Crude oil recovery from subterranean formations generally is classified as either primary, secondary or tertiary recovery. Primary recovery is dependent on the driving forces provided by the gases and connate waters existing in contact with the oil in the reservoir. Before the natural driving forces required for primary recovery are depleted, problems are sometimes encountered which prevent the removal of hydrocarbon from the formation itself or from the wells drilled into it. For example, materials such as paraffins, waxes, asphalts, clays and drilling fluids can accumulate in the formation and the well bore, restricting or blocking the flow of oil. To resolve this problem, methods have been developed which are aimed at dissolving or otherwise removing the flow-restricting materials. Representative of such methods are those described in U.S. Pat. Nos. 3,402,770 and 3,693,719 in which treatment chemicals, sometimes referred to as work-over fluids, are injected into the well bore and into the formation in the immediate vicinity of the well bore for a time sufficient to dissolve or dislodge flow-restricting materials. After this time, oil production under the natural driving forces of the formation can be resumed from the treated well. In U.S. Pat. No. 3,402,770, an organic solvent such as carbon disulfide is used in conjunction with ethylene glycol ethers. In U.S. Pat. No. 3,693,719, hydrolyzable, aprotic, halogenated organic compounds such as allyl chloride are used.
Another problem encountered during primary recovery is poor pumpability of viscous crude oils. Downhole emulsification methods, such as those described in U.S. Pat. Nos. 3,380,531 and 3,467,195 and by G. G. McClaflin et al. in Proceedings of the Society of Petroleum Engineers of AIME, SPE 10094, pp. 9-22 (1982), have been used to improve the pumpability of viscous hydrocarbons. According to these methods, chemicals such as nonionic surfactants, anionic surfactants and/or sodium hydroxide are injected as an aqueous solution into the well bore, as opposed to the formation itself, to form an oil-in-water emulsion near the pump. Because the viscosity of the emulsion is substantially lower than the viscosity of the unemulsified viscous crude oil, it can be pumped to the surface with conventional equipment at increased rates. This permits viscous hydrocarbon in the formation to continue to enter the well bore under primary driving forces where it can be emulsified and recovered.
Following primary recovery, secondary energy sources are required for continued production of oil from hydrocarbon-bearing formations. This usually entails injecting fluids into wells to drive oil out of the reservoir. Typical secondary oil recovery methods are water flooding and immiscible gas flooding. Over time, secondary recovery methods yield less and less oil. Eventually, only the injected fluids are recovered, leaving behind unrecovered oil as a residual oil saturation.
Tertiary oil recovery, also known as enhanced oil recovery (EOR), is recovery of oil which cannot be recovered by either primary or secondary methods. The aim of tertiary oil recovery methods is to reduce oil saturation. One method of achieving this aim is by reducing the viscosity of the oil remaining in the reservoir. Viscosity reduction is achieved through the application of heat or by the injection of fluids that act as solvents.
Tertiary oil recovery methods employing heat have been performed in single wells and are referred to as thermal "huff and puff" methods. A fire flood "huff and puff" method is described in U.S. Pat. No. 3,332,482. According to this method, in situ combustion is allowed to take place in a subterranean formation, thereby generating the heat necessary for viscosity reduction of heavy crude within the formation. Air injection into the well is eventually terminated to extinguish the fire after which the well is open to production.
Other thermal "huff and puff" methods have involved the injection of steam into a formation where it is allowed to soak for a time sufficient to lower the viscosity of the oil remaining in the reservoir. The reduction in viscosity permits the oil to be pumped back out the same well through which steam was injected. There have been numerous variations on the steam "huff and puff" enhanced oil recovery method by which oil/water-miscible solvents, surfactants, organosilicones, bases (e.g., sodium hydroxide or ammonium hydroxide) and demulsifiers have been injected together with the steam. Examples of such methods are disclosed in U.S. Pat. Nos. 3,454,095; 3,459,265; 3,618,666; 3,924,683; 3,951,457; and 3,782,472. As a substitute for steam, hot hydrocarbon vapors have been injected into reservoirs as described in U.S. Pat. Nos. 4,362,213 and 4,407,367.
Reported non-thermal, chemical tertiary oil recovery methods require at least two wells in the same formation, an injection well and a production well. Such methods are generally applicable to reservoirs containing oils of relatively low viscosities. Chemicals, such as nonionic or anionic surfactants in aqueous solutions, are injected in one well to penetrate the formation. As the chemicals move through the reservoir, microemulsions are formed providing low interfacial tensions between the pairs of coexisting equilibrium phases. The mixture is then pumped out of a production well drilled into the same formation. Such a tertiary oil recovery method is referred to as micellar or microemulsion flooding. The basic principle of the process is to reduce the interfacial tension of the trapped reservoir oil by injecting a surfactant solution to achieve a miscible or misciblelike displacement. Residual oil is then mobilized and captured at a production well [R. N. Healy and R. L. Reed, Soc. Petr. Eng. J., October 1974, 491 (1974); Soc. Petr. Eng. J., 17(2):129 (1977)].
In U.S. Pat. No. 3,474,864 a method is described in which a slug of nonionic surfactants such as ethoxylated alcohols and ethoxylated alkylphenols and/or anionic surfactants such as petroleum sulfonates in saline is injected into an injection well which floods through the formation displacing residual oil. The oil is recovered out of a production well. Adsorbed chemicals can be retrieved by flooding water of decreased salinity through the reservoir and recovering the chemicals from the water emerging out of the production well. Micellar flooding methods have also been performed by chasing the injected micellar slug with an aqueous solution of polymers, e.g., polysaccharides or polyacrylamides. This increases the viscosity of the water chase behind the micellar slug and enhances displacement of oil in the reservoir. [For a general discussion of micellar-polymer flooding, see Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., pp. 174-179 (1982).]